Alkylation, as the term is commonly used in the petroleum industry, is the reaction between an olefin and a branched chain paraffin to obtain a highly branched chain paraffin having a higher molecular weight than the isoparaffin employed as the initial reactant. Commercial processes using strong mineral acid catalysts alkylate isobutane with C.sub.3 -C.sub.4 olefins to form high octane liquid products which distill in the gasoline range, 35.degree.-210.degree. C.
Two well known commercial processes are in liquid phase and use strong acids, i.e., hydrofluoric (HF) and sulfuric acid (H.sub.2 SO.sub.4). Although these processes have been successful from both an alkylate yield and a quality standpoint, there are some inherent disadvantages using strong acids in liquid phase. Major disadvantages are threats to operator safety and a high risk of environmental pollution as a result of accidental acid release and from spent acid disposal. This has prompted the exploration of solid catalyst processes for alkylation.
A large variety of solid catalysts, including amorphous and crystalline materials, have been demonstrated to be effective for use as alkylation catalysts. However, a major drawback of using a solid catalyst is a rapid loss in catalyst activity, thus requiring frequent regeneration of the catalyst. A number of regeneration methods including solvent wash, use of ultrasonics, conventional oxygen burn, etc. have been proposed.
Commercial alkylation processes which use solid catalysts generally incorporate an alkylation cycle and a catalyst regeneration cycle. In the alkylation cycle, the catalyst is in contact with the hydrocarbon feed. This cycle is also known as the "on-oil" portion of the operating cycle. During the on-oil portion of the operating cycle, the activity of the catalyst gradually declines due to the build-up of carbonaceous deposits, or coke, on the catalyst. During the catalyst regeneration cycle, the catalyst is taken out of contact with the hydrocarbon feed and the catalyst is regenerated.
In the semi-regenerative process, during the on-oil portion of the operating cycle, the entire unit is operated by gradually and progressively adjusting the temperature and feed rate to maintain yield and selectivity. At a predetermined activity level the entire unit is shut down for regeneration of the catalyst. After regeneration, the unit is put back in the on-oil cycle.
In a cyclic regeneration process, each individual reactor of a plurality of reactors is capable of being individually isolated. In effect, during the regeneration cycle, a reactor is swung out of line by various manifolding arrangements, such as by motor operated valving and the like. The catalyst is regenerated to remove the coke deposits while the other reactors remain on stream. A "swing reactor" temporarily replaces the reactor which is swung out of line, until regeneration of the catalyst is complete.
The net result in either type of regeneration process is, however, the same. The coke must be oxidatively burned from the catalyst at temperatures ranging from about 400.degree. C. to about 800.degree. C., and the higher the required temperature, inter alia, the greater the damage to the catalyst.
Attempts have been made to regenerate zeolite catalysts at low temperature, as disclosed by (1) Copperthwaite, R. G. et al., J. Chem. Soc., Chem. Commun. 1985, p 644-645; (2) Copperthwaite, R. G. et al., J. Chem. Soc., Faraday Trans. 1, 1986, 82, p 1007-1017; and (3) Hutchings, G. J. et al., Applied Catalysis, 34, 1987, p 153-161. In attempts to regenerate a 1/16 inch zeolite Y type of catalyst extrudate (LZY82.RTM., Union Carbide), as described by Hutchings et al., upon breaking the catalyst particles, they observed a black core of coked catalyst surrounded by a white layer of partially regenerated catalyst. More carbon remained on the catalyst when it was regenerated with ozone than remained on a similar catalyst regenerated with oxygen. In the regeneration of ZSM-5 powder, as described in the two Copperthwaite et al. publications, generally similar results were obtained. It would be very advantageous, and indeed a need exists, for a low temperature process which completely regenerates coked solid alkylation catalysts to perform in a manner that is essentially equivalent to that of fresh catalyst.